The long-term goal of this project is to develop a new technology to improve the screening for and diagnosis of breast cancer. Non-invasive electrical impedance measurements made with a hand-held robe have been shown to improve the specificity and sensitivity of mammography for breast tumor diagnosis in patients with ambiguous mammograms. This non-invasive technology poses no known risks to the subject, and provides a new diagnostic parameter to assess suspicious anomalies. This new technology, called Electrical Impedance Tomography, makes images of the interior of the body From measurements made at its surface. Small electric currents are passed through the body using electrodes applied to the skin; the resulting voltages are then measured and used by mathematical algorithms to reconstruct the values of the electrical conductivity and permittivity of the underlying structures. The electrical properties of breast tumor tissue differ by a factor of 5 -10 from those of surrounding, normal tissue. Benign breast lesions are also electrically different from malignancies. By Forming images of the electrical conductivity of the breast, tumors may be detected and differentiated. When this is done using several different frequencies of electrical current, and by measuring both conductivity and permittivity, the technique is called tissue spectroscopy, and further diagnostic information may be elucidated. We propose to determine the feasibility of improving the diagnostic ability of mammography by combining a mammography system with an adaptive current tomography (ACT) system in order to make simultaneous, in-registration images of breast electrical properties using data collected from two arrays of radiolucent electrodes applied to the mammogram plates. This is a proposal to build the required electronic instruments and electrode arrays, and to write improved algorithms to reconstruct conductivity and permittivity images from the resulting voltages and currents. It will operate at frequencies between 300 Hz and 1 MHz, using 64 electrodes, and make and display 20 frames/sec. The system will be designed for clinical use, and will be able to assess the pulsatility of blood volume, which may indicate malignancy. It will be applied in a study of normal human subjects, to determine basic operational procedures for its use. It will then be used in a clinical examination of patients undergoing biopsy for breast cancer diagnosis. Images will be made simultaneously and in register by mammography and by electrical impedance tomography. Comparison of both images with biopsy results will be made. The use of impedance images and spectroscopy for the diagnosis of breast cancer may then be able to be responsibly assessed.